• Guillaume Knispel's avatar
    ipc: optimize semget/shmget/msgget for lots of keys · 0cfb6aee
    Guillaume Knispel authored
    ipc_findkey() used to scan all objects to look for the wanted key.  This
    is slow when using a high number of keys.  This change adds an rhashtable
    of kern_ipc_perm objects in ipc_ids, so that one lookup cease to be O(n).
    
    This change gives a 865% improvement of benchmark reaim.jobs_per_min on a
    56 threads Intel(R) Xeon(R) CPU E5-2695 v3 @ 2.30GHz with 256G memory [1]
    
    Other (more micro) benchmark results, by the author: On an i5 laptop, the
    following loop executed right after a reboot took, without and with this
    change:
    
        for (int i = 0, k=0x424242; i < KEYS; ++i)
            semget(k++, 1, IPC_CREAT | 0600);
    
                     total       total          max single  max single
       KEYS        without        with        call without   call with
    
          1            3.5         4.9   µs            3.5         4.9
         10            7.6         8.6   µs            3.7         4.7
         32           16.2        15.9   µs            4.3         5.3
        100           72.9        41.8   µs            3.7         4.7
       1000        5,630.0       502.0   µs             *           *
      10000    1,340,000.0     7,240.0   µs             *           *
      31900   17,600,000.0    22,200.0   µs             *           *
    
     *: unreliable measure: high variance
    
    The duration for a lookup-only usage was obtained by the same loop once
    the keys are present:
    
                     total       total          max single  max single
       KEYS        without        with        call without   call with
    
          1            2.1         2.5   µs            2.1         2.5
         10            4.5         4.8   µs            2.2         2.3
         32           13.0        10.8   µs            2.3         2.8
        100           82.9        25.1   µs             *          2.3
       1000        5,780.0       217.0   µs             *           *
      10000    1,470,000.0     2,520.0   µs             *           *
      31900   17,400,000.0     7,810.0   µs             *           *
    
    Finally, executing each semget() in a new process gave, when still
    summing only the durations of these syscalls:
    
    creation:
                     total       total
       KEYS        without        with
    
          1            3.7         5.0   µs
         10           32.9        36.7   µs
         32          125.0       109.0   µs
        100          523.0       353.0   µs
       1000       20,300.0     3,280.0   µs
      10000    2,470,000.0    46,700.0   µs
      31900   27,800,000.0   219,000.0   µs
    
    lookup-only:
                     total       total
       KEYS        without        with
    
          1            2.5         2.7   µs
         10           25.4        24.4   µs
         32          106.0        72.6   µs
        100          591.0       352.0   µs
       1000       22,400.0     2,250.0   µs
      10000    2,510,000.0    25,700.0   µs
      31900   28,200,000.0   115,000.0   µs
    
    [1] http://lkml.kernel.org/r/20170814060507.GE23258@yexl-desktop
    
    Link: http://lkml.kernel.org/r/20170815194954.ck32ta2z35yuzpwp@debixSigned-off-by: default avatarGuillaume Knispel <guillaume.knispel@supersonicimagine.com>
    Reviewed-by: default avatarMarc Pardo <marc.pardo@supersonicimagine.com>
    Cc: Davidlohr Bueso <dave@stgolabs.net>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Manfred Spraul <manfred@colorfullife.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Cc: Serge Hallyn <serge@hallyn.com>
    Cc: Andrey Vagin <avagin@openvz.org>
    Cc: Guillaume Knispel <guillaume.knispel@supersonicimagine.com>
    Cc: Marc Pardo <marc.pardo@supersonicimagine.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    0cfb6aee
msg.c 23.7 KB